1. Field of the Invention
The present invention relates to the manufacturing of semiconductor devices. More particularly, the present invention relates to a heater assembly for heating a wafer.
2. Description of the Related Art
Generally, a semiconductor device is manufactured by forming a minute electronic circuit pattern on a substrate. The circuit pattern comprises a great number of electronic elements and wiring connecting the electronic elements.
More specifically, a silicon wafer, namely, a small thin circular slice of pure silicon, is produced from an ingot of crystalline silicon. Then, an electronic circuit is formed on a surface of the wafer (wafer fabrication, FAB process), the wafer is cut into a plurality of individual chips, and each chip is combined with a lead frame. An operating test is then performed on the chip to ensure that the semiconductor device is fully functional.
In the FAB process, a thin film is formed on the surface of the wafer, and the thin film is patterned to form an electronic circuit for performing a specific function. Hence, if the thickness of the thin film is not uniform over the entire surface of the wafer, a residual stress occurs on the surface of the wafer. As a result, the integrated circuit may not be formed properly.
The rate at which the material constituting the thin film layer is deposited on the wafer is greatly dependent on the temperature of the wafer. Specifically, assuming all other deposition conditions to be the same, the thin film is formed more quickly and hence, more thickly, on a wafer surface at a high temperature than at a low temperature. The deposition process of forming the thin film is carried out while the wafer is repeatedly heated and cooled. Accordingly, the temperature varies across the wafer surface, especially between a peripheral portion and a central portion of the wafer. Thus, the thin film may be formed non-uniformly over the wafer surface. Stress will occur at the boundary where the thickness of the film changes. The stress deforms the thin film, which phenomenon is referred to as xe2x80x9ca slip phenomenonxe2x80x9d. Consequently, an IC patterned on the deformed thin film may lose its functional stability. That is, temperature uniformity is an essential factor for fabricating a semiconductor device having functional stability. In consideration of the recent trend in semiconductor technology for devices having higher degrees of integration acquired through reducing the critical dimension of the circuit patterns thereof, the temperature uniformity of the wafer surface is increasing in importance as a processing condition to be established during the semiconductor device manufacturing process.
Chemical vapor deposition (CVD) is the method usually used for forming the thin film in the semiconductor device manufacturing process. Thermal CVD is being used more frequently for forming such thin films. In thermal CVD, material is deposited by means of heat-induced chemical reactions of reactant gases supplied to a surface of a heated wafer. The thermal CVD process are classified into atmospheric pressure CVD (APCVD) and low pressure CVD (LPCVD) processes on the basis of the pressure in the CVD apparatus. LPCVD is especially suitable for depositing a metal silicide having a high melting point to form a polycide that is generally used as a wiring material of a highly integrated circuit device.
The LPCVD apparatus includes a susceptor for supporting and fixing a wafer on an upper surface thereof, and a heater disposed below the susceptor for providing heat to the susceptor. That is, the heat generated by the heater radiates to the susceptor and is conducted from the susceptor to the wafer. Therefore, the temperature of the wafer surface is dependent on the amount of heat conducted from the susceptor, and the conducted heat from the susceptor is mainly dependent on the amount of heat radiating from the heater. That is, the temperature of wafer surface is mainly dependent on the amount of heat radiating from the heater.
However, even though equal amounts of heat radiate to the peripheral portion and the central portion of the wafer from the heater, the surface temperature at the peripheral portion of the wafer is lower than that at the central portion of the wafer because a significant amount of heat is lost at a side surface of the peripheral portion of the wafer whereas most of the heat is conserved at the central portion of the wafer. Consequently, the surface temperature of the wafer is much lower at the peripheral portion than at the central portion of the wafer.
Various attempts have been made to structure the heater to decrease the temperature difference between the various surface portions of the wafer. For example, U.S. Pat. No. 6,031,211 entitled xe2x80x9cZONE HEATING SYSTEM WITH FEEDBACK CONTROL SYSTEMxe2x80x9d discloses a heating system and method for producing temperature uniformity at the surface of the wafer. The disclosed heating system includes a plurality of heating sections that are controlled independently to generate different amounts of heat used for heating respective portions of the wafer. Furthermore, a heater assembly of a GENUS 7000 (trade name) CVD apparatus made by GENUS Co. Ltd. U.S.A, which is a widely used thermal CVD apparatus, includes an inner heater for heating a central portion of a susceptor and an outer heater for heating a peripheral portion of the susceptor. The inner heater and outer heater are discrete from each other and are respectively controlled to generate more heat at the peripheral portion than at the central portion. Accordingly, heat loss at the side surface of the peripheral portion of the wafer is compensated for by the outer heater, in an attempt to produce temperature uniformity on the surface of the wafer.
However, the dual heater system does not produce such temperature uniformity even when the outer heater is generating more heat than the inner heater.
FIG. 1 is a schematic cross-sectional view of the conventional dual heater assembly of the GENUS 7000 thermal CVD apparatus made by GENUS Co. U.S.A. FIG. 2 is a schematic plan view of the dual heater assembly. Referring to FIGS. 1 and 2, the conventional dual heater assembly 90 includes a susceptor 40 for supporting a wafer 50, a plurality of heaters 10 disposed below the susceptor 40 for providing heat to the susceptor 40, an electrical power source for supplying electric current to the heaters 10 and a support 30 for supporting the heaters 10.
The heaters 10 include an outer heater 12 for heating a peripheral portion of the susceptor and an inner heater 14 for heating an inner portion of the susceptor. The outer heater 12 and the inner heater 14 are separated from each other by a space 16 for preventing heat transfer between the outer heater 12 and the inner heater 14. In addition, the outer heater 12 and the inner heater 14 are controlled to operate independently. Each of the heaters 10 is made of a thin plate of graphite. Heat is generated due to the internal resistance of the heaters 10 when the electric current is supplied to the heaters 10. The electrical power source includes a first source (not shown) for providing current to the outer heater 12 and a second source 20 for providing current to the inner heater 14.
The second source 20 comprises a connection member 24 for guiding electric current from an external power source to the inner heater 14, a lead member 22 which is connected to an input terminal formed on the bottom surface of the inner heater 14, and a controller (not shown) for controlling the electric current supplied through the connection member 24 and lead member 22 according to a surface temperature of the wafer 50. The lead member 22 comprises a corrosion-resistant and heat-resistant material and is screwed onto the input terminal.
The support 30 is made of quartz, which is corrosion-resistant to acid or alkali materials except hydrogen fluoride and thus, is very chemically stable. Hence, the support 30 is not easily corroded by deposition gas or other pollutants, and operates as an insulator.
When electric current is provided to the heaters 10 from the external power source via the electrical power source, heat generated from the graphite heaters 10 radiate to the susceptor 40. Subsequently, heat is conducted from the susceptor 40 to the wafer 50 disposed on top of the susceptor 40, whereby the wafer 50 is heated. In heating the wafer 50 as described above, the outer heater 12 is controlled to generate more heat than the inner heater 14.
Nonetheless, experiments show that the temperature of the wafer surface varies from the central portion to the peripheral portion of the wafer.
FIG. 3 shows the temperature distribution across the surface of a wafer heated by the conventional dual heater system. The temperature was measured at 25 spots on a test wafer heated by controlling the outer heater 12 and the inner heater 14 to produce a temperature difference of 20xc2x0 C. FIG. 4 depicts the temperature profile of the wafer surface using the temperature data shown in FIG. 3
Referring to FIG. 3, even though the outer heater 12 was controlled to generate more heat than the inner heater 14, the temperature of wafer surface is relatively high at the central portion of the wafer and relatively low at the peripheral portion of the wafer. That is, the results shown in FIG. 3 indicate that the dual heater system cannot make the temperature of the wafer surface sufficiently uniform. Furthermore, since the input terminals of the inner and outer heaters are disposed at locations laterally offset with respect to the center of the wafer, the temperature at a region of the wafer surface closest to the input terminals (xe2x80x9cregion IIxe2x80x9d in FIG. 3) is higher than that of a region of the wafer surface remote from the input terminals (xe2x80x9cregion Ixe2x80x9d in FIG. 3). Therefore, the temperature profile is skewed towards the input terminal, as shown in FIG. 4.
The variation in the temperature across the wafer surface thus causes the thickness of the thin film formed on the wafer surface to be non-uniform. This, in turn, can cause a variety of processing defects.
FIG. 5 shows the sheet resistance profile of the surface of the wafer disposed on the conventional heater assembly having a dual heater system. The sheet resistance is measured at a number of spots on the surface of the wafer, and the sheet resistance profile is drawn by connecting the spots where the resistance values are the same. It is noted that the temperature profile and the sheet resistance profile have similar shapes.
When tungsten silicide is deposited to form a thin film, the sheet resistance per unit area has a relationship according to equation (1) as follows:                               R          s                =                  ρ          t                                    (        1        )            
wherein Rs is the sheet resistance, xcfx81 is the bulk resistivity and t is the thickness of thin film. The sheet resistance can be easily measured using various measuring instruments. Therefore, the thickness of the film at the spot where the sheet resistance is measured can be easily calculated by using equation (1).
Also, referring to FIG. 6, the connection member 24 for conducting electric current to the inner heater 14 passes through the support 30 and contacts the lead member 22, which is connected to the input terminal 14a. The inner heater 14 and the input terminal are enclosed within an insulating layer 18 so as to be prevented from being eroded by deposition gas, by-products produced during a deposition process and ionized fluorine (Fxe2x88x92) produced during the rinsing of the CVD apparatus.
However, the lead member 22 is mechanically joined with, e.g., screwed to, the input terminal 14a of the inner heater 14. Thus, deposition gas and by-products produced during the deposition process may infiltrate the insulating layer 18 through chinks in the mechanical joint between the lead member 22 and the input terminal. Consequently, the lead member 22 is not only expanded due to heat from the heater 10, but also is eroded by the deposition gas and by-products produced during the deposition process. Furthermore, the lead member 22 is eroded by ionized fluorine (Fxe2x88x92) of a solution used to rinse the CVD apparatus. The thermal expansion and erosion of the lead member 22 cause the lead member 22 to crack. Therefore, electric current cannot reach the inner heater 14. Accordingly, the entire heater assembly must be changed due to a mere defect in the lead member 22.
As described in the above, even though the outer heater is independently controlled to generate more heat than the inner heater does, the temperature is not sufficiently uniform across the wafer surface. Accordingly, the thickness of a thin film formed on the wafer surface is non-uniform. In addition, the lead member of the system is prone to cracking, i.e., the useful life of the heater assembly is short.
Therefore, one object of the present invention to provide a heater assembly that minimizes the temperature difference between a peripheral portion and a central portion of the wafer. Another object of the present invention is to provide a heater assembly having a long useful life.
The heater assembly comprises a first support, e.g., a susceptor, for supporting the wafer, a heater including a unitary body of electrically resistive material for generating heat by electrical resistance, and a plurality of heat blocks dividing the upper surface of the unitary body into a plurality of heating sections, a second support disposed under and supporting the resistive heater, and an electrical power source for supplying electric current to the resistive heater.
The widths of the heating sections increase towards the center of the heater to such an extent that the electrical resistance of the peripheral portion of the heater is higher than that of the central portion of the heater.
The unitary body of resistive material has the shape of a disc, and the heating assembly further comprises an input terminal and an output terminal disposed along a diameter of the disc and connected with the electrical power source. The heating sections are contiguous to each other to form a single path for electric current between the input terminal and the output terminal.
The heat blocks include a circular outer block member extending along the outer periphery of the disc, a first block member disposed radially inwardly of the outer block member such that a first heating section is defined between the outer block member and the first block member, and a second block member disposed radially inwardly of the first block member such that a second heating section is defined between the first block member and the second block member, and a third heating section is defined radially inwardly of the second block member.
The first block member has a plurality of arcuate sections lying along a first circle and spaced from one another so as to provide a first opening and a second opening therebetween. Each of the first and second openings subtends a first angle xcex81 in a circumferential direction of the disc from an imaginary line connecting the input terminal and the output terminal. Hence, the first and second openings are disposed symmetrically to one another with respect to the center of the heater.
The second block member has a plurality of arcuate sections lying along a second circle and spaced from one another so as to provide a third opening and a fourth opening therebetween. Each of the third and fourth openings subtends a second angle xcex82 in the circumferential direction of the disc from the imaginary line connecting the input terminal and the output terminal. Hence, the third and fourth openings are also disposed symmetrically to one another with respect to the center of the heater.
First, second, third and fourth guide members of electrically insulating material are also integrated into the upper surface of the body of the heater so as to guide electric current to and from the heating sections at the upper surface of the heater. The first guide member connects the outer block member with the first block member at a proximal end of the first opening in the first heat block member. The fourth guide member connects the outer heat block member with the first block member at a proximal end of the fourth opening such that the first and fourth guide members are symmetrical to each other with respect to the center of the heater. The second guide member connects the second block member at a proximal end of the third opening with the first block member at a distal end of the first opening. The third guide member connects the first block member at a distal end of the third opening with the second block member at a proximal end of the third opening such that the second and third guide members are symmetrical to one another with respect to the center of the heater.
The electrical power source includes an electrical lead connected to the input terminal for conducting electric current to the heater, and an electrical connector for electrically connecting the lead to an external (driving) power source. The lead extends from the lower surface of the resistive heater downwardly through the heater support and into contact with the connector at a bottom portion of the support.
According to the present invention, the electrical resistance is higher at peripheral portion than at central portion of the heater. Accordingly, the peripheral portion of the heater generates more heat than the central portion. Consequently, the temperature difference between the peripheral portion and the central portion of the wafer can be minimized, and a thin film layer having a uniform thickness can be formed by a deposition process. Furthermore, the lead is not likely to crack and thus, the costs associated with maintaining the CVD apparatus can be kept low.